284 EDUARD TJHLENHUTH 



data, uniformly point to the one conclusion: There is in meta- 

 morphosis of an organ (i.e. in the development of the adult 

 characteristics of an organ from the larval structure of this 

 organ) one factor involved, which is not contained in this organ 

 nor can it be produced by this organ. 



4. In the same direction seem to point the experiments on 

 feeding thyroid-gland to Amphibian larvae and on extirpation 

 of the thyroid and hypophysis in amphibian larvae. These 

 experiments demonstrate at least that the influence of certain 

 substances which are introduced or contained primarily in the 

 body of the animal and not in the organs in question, is in some 

 way able to reach these organs; they demonstrate beyond any 

 doubt that metamorphosis can be induced by an external factor 

 not primarily contained in the organs themselves. 



It seems interesting to us that a certain resemblance doubtlessly 

 exists between the action of thyroid substance and that of the 

 factor disclosed in the grafting experiments. First, they are 

 both external to the organs and second, both are non-specific. 

 Third, it seems that thyroid, like the metamorphosis factor, 

 acts only by inducing the processes of metamorphosis; if these 

 processes have once been induced by the action of the substance 

 they will continue even in its absence. Kahn, for instance, 

 stated that a single feeding with thyroid is enough to induce 

 metamorphosis and that without any further feeding metamor- 

 phosis may be completed in a short time. We have seen that 

 in eyes in which metamorphosis has once been induced it will 

 be completed even if the eyes are removed from the influence of 

 the agent and transferred to young larvae in which this agent 

 has not yet developed Fourth: it seems that in very young 

 animals thyroid must act longer than in older animals in order 

 to induce metamorphosis. 8 This corresponds with the fact that 

 organs grafted from young to old larvae in which metamorphosis 

 would occur a few days after operation, metamorphose sometime 

 after the host instead of simultaneously with the host. 



In our experiments it was not possible to demonstrate actually 

 that the metamorphosis of all three organs examined, i.e., eyes ? 



8 T. F. Gudernatsch, 1916, p. 358. 



